1. Field of the Invention
The present invention relates to crosslinked elastomeric polymers having a relatively low density of crosslinks, that is, a partially crosslinked unsaturated elastomeric polymer.
2. Description of Information Disclosures
Crosslinked elastomers are known in which the molecular chains are crosslinked at the sites of the carbon to carbon double bond, of the uncrosslinked initial polymer, as well as at the site of other functional groups present in the uncrosslinked polymer, such as, for example at the site of chemically-bound halogen.
Partially crosslinked polymers are known such as those disclosed in U.S. Pat. Nos. 2,781,334 and 2,729,626 in which the polymers are prepared by copolymerizing an isoolefin with a conjugated diolefin and a divinyl benzene. Such polymers require the use of divinyl benzene in its preparation as well as the use of the isoolefin and the conjugated diolefin. A brochure of Polysar Corporation discloses Polysar.RTM. Butyl Terpolymers such as XL 10,000 (previously XL-20), including Polysar.RTM. Butyl Terpolymer XL 40302 which is described as a halogenated type (printed in Canada, 1988).
Halogenated butyl rubber having a low level of halogen e.g., chlorine content of about 0.5 mole percent (about 0.3 wt.%) and a low level of non-halogenated isoprene unsaturation are disclosed in a paper entitled "Chlorobutyl Designed for Curing Members" authored by J. V. Fusco, I. J. Gardner, P. Hous and D. S. Tracey presented at a meeting of the Rubber Division, American Chemical Society, Mexico City, Mexico, May 9-12, 1989.
Numerous patents disclose halogenated butyl rubber containing various amounts of chemically bound halogen, for example, U.S. Pat. No. 2,631,984 discloses brominated butyl rubber containing at least 0.5 weight percent bromine; U.S. Pat. No. 2,732,354 discloses chlorinated butyl rubber containing from 0.4 to 2.3 weight percent chlorine; U.S. Pat. No. 3,099,644 discloses a process for the continuous halogenation of butyl rubber to produce a halogenated rubber containing 1.13 weight percent chlorine. U.S. Pat. No. 2,732,354 discloses chlorinated butyl rubber containing from about 0.4 to 2.3 weight percent of chlorine. U.S. Pat. No. 2,944,578 describes compositions comprising chlorinated butyl rubber in which the butyl rubber has been substitutively chlorinated to contain at least 0.5 weight percent chlorine; FIG. II of this patent appears to include a data point at about 0.6 wt.% chlorine. U.S. Pat. No. 3,099,644 discloses a process for the continuous halogenation of butyl rubber to produce a halogenated rubber containing 1.13 weight percent chlorine. U.S. Pat. No. 2,943,664 discloses adhesive compositions for tire cords comprising chlorinated butyl rubber containing at least 0.5 percent by weight of combined chlorine. U.S. Pat. No. 2,964,489 discloses a process for the production of tubeless tires and curing bladders containing 0.5 weight percent chlorine. The chlorine content of known chlorinated butyl rubber is typically defined by a formula, as shown, for example, in U.S. Pat. No. 2,964,489 (column 2, lines 50-60) and expressed as being "at least 0.5 wt. percent (preferably at least about 1.0 wt. percent) combined chlorine." In a restatement of the chlorine concentration, it is stated that there should be "at least 0.2 and preferably at least about 0.5 weight percent of combined chlorine in the polymer." (id, lines 60-62).
U.S. Pat. No. 4,130,534 discloses elastoplastic compositions comprising a blend of thermoplastic crystalline polyolefin resin and a butyl rubber, crosslinked to the extent that it is at least 90 percent gelled. It defines butyl rubber (at column 5, lines 17-34) to include "copolymers of isobutylene and isoprene containing 0.5 to 10, more preferably 1 to 4 percent by weight isoprene and said copolymers halogenated with from 0.1 to about 10, preferably 0.5 to 3.0 weight percent chlorine or bromine which chlorinated copolymer is commonly called chlorinated butyl rubber".
U.S. Pat. No. 3,758,643 and U.S. Pat. No. 3,862,106 (W. K. Fischer) disclose a partially crosslinked EPDM polymer having a gel content of at least 30 percent but less than 90 percent by weight as measured by immersion in cyclohexane for 48 hours at 73.degree. F.
It is known to perform melt phase reaction of polymers in continuous flow devices, such as extruder-reactors. See, for example, U.S. Pat. Nos. 4,513,116; 4,548,995; 4,554,326, and 4,563,506, the teachings of which are hereby incorporated by reference.
U.S. Pat. No. 4,594,390 discloses the use of an extruder to prepare thermoplastic compositions comprising a blend of plastic and cured rubber.
There is field of use for butyl rubber wherein the rubber itself is required to be uncured, but where the inherent properties of uncured rubber (cold flow, low green strength) represent a drawback. Such uses include those where butyl and halobutyl are the polymeric material in easily processed formulations suitable for mastics, pipe wrap, sealants, cable fillers etc. Formulations based on such uncured rubbers tend to flow in service, and therefore, it would be useful to employ in such formulations butyl or halobutyl rubber which has been partially cured.
The disadvantages associated with known partially cured butyl rubber derive from the manner in which such polymers are produced and the means for crosslinking which are employed. For example, known partially crosslinked butyl is produced by adding to the butyl compound, under curing conditions, a curative such as a sulphur or phenol resin system in an amount which is less than the amount required to give crosslinking at all of the available crosslinking sites. The result is a vulcanized rubber which is crosslinked through some of the unsaturation sites in the copolymer backbone. However, because such unsaturation sites are relatively numerous, and because the process is limited by dispersion of the curatives in the polymer, it is extremely difficult to control the crosslinking. The result is a product with a reduced unsaturation and with inconsistent product quality deriving from the failure to control the extent and location of crosslinking within and between the polymer chain. Similar problems arise when the starting polymer, prior to crosslinking, is known halobutyl rubber, since in this case the halogen atoms carried on the isoprene units also contribute to the crosslinking mechanism, making control even more difficult.
An alternative way to obtain partially cured butyl rubber is to introduce a crosslinking agent such as divinyl benzene into the reactor in which the copolymerization is carried out. Again, though, the degree and distribution of crosslinking along the copolymer chains is difficult to control, and the product, therefore, suffers from variability in its properties.
It has now been found that elastomeric polymers can be produced which have a controlled relatively low degree of crosslinking, thereby constituting useful products which can be used where uncured rubbers have been used but without the disadvantages of uncured rubber. The partially crosslinked elastomeric polymers of the present invention can be further processed in conventional rubber processing equipment and are useable in such formulations as adhesives, sealants, etc. The partially crosslinked elastomeric polymers of the invention possess elastomeric as well as thermoplastic properties.